The present investigation will elucidate the nature of the microbiota associated with destructive periodontal diseases of the young adult aged 20-35. Predominant organisms from such lesions and lesions of other destructive forms of disease will be tested for their pathogenic potential in gnotobiotic rats and hamsters. Organisms whose classification is uncertain will be extensively characterized: attempts will be made to develop functional classification schemes. If specific organisms appear to be associated with specific disease processes, efforts will be made to devise selective media for their isolation and enumeration.